Wrap a tape measure around your ribcage, following the base of your bra band - pull taut so there's no dip in the tape, but not so tight that you create an indent. This number, in inches, is your band measurement. If the number is odd, round up to the nearest even.

Step 3: Bust Measurement

Move the tape around the fullest part of your bust, typically at the center of your nips, wrap evenly around your back, holding taut without any dipping. Take this measurement in inches, this is your bust measurement.

Step 4: Bra Size

Take your bust measurement (second one) and subtract your band measurement (first one). Each inch difference between your band and bust measurements equals one cup size. The band measurement and cup size combined are your underwire bra size.